Operations in Polar Regions and where Ice is Prevalent
	See also: 'Ice Accumulation on Ships'
	POLAR REGIONS
	The polar environment
	In high latitudes, directions change fast with movement of the observer. Near the poles, meridians converge, and excessive longitudinal curvature renders the meridians and parallels impracticable for use as navigational references. All time zones meet at the poles, and local time has little significance. Sunrise and sunset, night and day, as they are known in the temperate regions, are quite different in polar regions. At the poles the sun rises and sets once a year, slowly spiralling for three months to a maximum altitude of 23° 27' and then decreasing in altitude until it sets again three months later. The Moon rises once each month and provides illumination when full, though sometimes the aurora (See also: 'The Aurora Borealis Show') gives even more light; and the planets rise and set once each sidereal period (12 years for Jupiter, 30 years for Saturn). Fog is most frequent when the water is partly clear of ice. Low cloud ceilings are prevalent. 'Whiteouts' occur from time to time, when daylight is diffused by multiple reflection between a snow surface and an overcast sky, so that contrasts vanish and neither the horizon nor surface features can be distinguished. All these conditions, combined with the ice itself, add to the difficulties of navigation.
	Charts
	Polar charts are baled largely on aerial photography which may be without proper ground control, except in a relatively few places where modern surveys are available, e.g. in the approaches to bases and similar frequented localities. Even then, the conditions under which these surveys have been carried out are such that their accuracy is unlikely to be similar to that of work done in more clement dimes. For these reasons the geographical positions of features may be unreliable and, even when they are correctly placed relative to adjacent features, considerable errors may accumulate when they are separated by appreciable distances. In general soundings, topography and all navigational information are sparse in most polar regions. Visual and radar bearings, unless of observed objects which are close, require to be treated as great circles. If used on a Mercator chart, bearings should be corrected for half-convergency in the same way as radio bearings. See Admiralty List of Radio Signals Vol 2. See the ALRS Publications. Natural landmarks are plentiful in some areas, but their usefulness is restricted by the difficulty in identifying them, or locating them on the chart. Along many of these coasts the various points and inlets bear a marked resemblance to each other. The appearance of a coast is often very different when many of its features are marked by a heavy covering of snow or ice than when it is ice-free.
	Compasses
	The gyro compass losses all horizontal directive force as the poles are approached and is thought to become useless at about 85° of latitude. It is generally reliable up to 70° but thereafter should be checked by azimuths of celestial bodies at frequent intervals (about every 4 hours and more frequently in higher latitudes). Frequent changes of course and speed and the impact of the vessel on ice introduce errors which are slow to settle out. The magnetic compass is of little value for navigation near the magnetic poles. Large diurnal changes in variation (as much as 10°), attributed to the continual motion of the poles, have been reported. In other parts of the polar regions, however, the magnetic compass can be used, provided that the ship has been swung and the compass adjusted in low latitudes, and again on entering high ones. Frequent comparisons of magnetic and gyro compasses should be made and logged when azimuth checks are obtained.
	Sounders
	The echo sounder should be run continuously to detect signs of approaching shoal water, though in many parts of the polar regions depths change too abruptly to enable the Mariner to rely solely on the sounder for warning. In some better sounded areas, the depths may give an indication of the ship's position, or of the drift of the ice, and in these areas ships should make use of all enforced stops to obtain a sounding. Working in drift ice the echo sounder trace may be lost due to ice under the ship or hull noises, so, if necessary, the ship should be slowed to obtain a sounding.
	Sights
	The Mariner cannot rely on obtaining accurate celestial observations. For much of the navigational season clouds hide the sun, and long days and short nights in summer preclude the use of stars for observations. In summer when, apart from the moon at times, only the sun can be used for observations, transferred position lines must be used, and as accurate dead reckoning in ice is impossible, the accuracy of the resulting positions must always be questioned. The best positions are usually obtained from star observations during twilight. As the latitude increases twilights lengthen, but with this increase come longer periods when the sun is just below the horizon and the stars have not yet appeared. In polar regions the only celestial body available for observations may not exceed the altitude of 10° for several weeks on end, so that, contrary to the usual practice, observations at low altitudes must be accepted. Most celestial observations in polar regions produce satisfactory results, but in high latitudes the navigator should be on the alert for abnormal conditions.
	APPROACHING ICE
	Readiness for ice
	Experience has shown that ships that are not ice-strengthened and with a speed in open water of about 12 knots often become firmly beset in light ice conditions, whereas an adequately powered ice-strengthened ship should be able to make progress through 6/10 to 7/10-first-year ice. The engines and steering gear of any ship intending to operate in ice must be reliable and capable of quick response to manoeuvring orders. The navigational and communications equipment must be equally reliable and particular attention should be paid to maintaining radar at peak performance. Ships operating in ice should be ballasted and trimmed so that the propellor is completely submerged and as deep as possible, but without excessive stem trim which reduces manoeuvrability. If the tips of the propellor are exposed above the surface or just under the surface, the risk of damage due to the propellor striking ice is greatly increased. Ballast and fresh water tanks should be kept not more than 90 per cent full to avoid risk of damage to them from expansion if the water freezes. Good searchlights should be available for night navigation, with or without icebreaker escort.
	Signs of icebergs
	Caution. There are no infallible signs of the proximity of an iceberg. Complete reliance on radar or any of the possible signs can be dangerous. The only sure way is to see it. Unreliable signs. Changes of air or sea temperature cannot be relied upon to indicate the vicinity of an iceberg. However, the sea temperature in the North Atlantic, if carefully watched, will indicate when the cold ice-bearing current is entered. Echoes from a steam whistle or siren are also unreliable because the shape of the iceberg may be such as to prevent any echo, and also because echoes are often obtained from fog banks. Sonar has been used to locate icebergs, but the method is unreliable since the distribution of water temperature and salinity, particularly near the boundary of a current, may produce such excessive refraction as to prevent a sonar signal from reaching the vessel or iceberg. Likely signs. The following signs are useful when they occur, but reliability cannot be placed on their occurrence. In the case of large Antarctic icebergs, the absence of sea in a fresh breeze indicates the presence of ice to windward if far from the land. When a growler breaks away, or ice otherwise cracks and falls into the sea, it produces a thunderous roar, or sounds like the distant discharge of guns. The observation of growlers or smaller pieces of detached ice is an indication that an iceberg is in the vicinity, and probably to windward; an iceberg may be detected in thick fog by this means. When proceeding at slow speed on a quiet night, the sound of breakers may be heard if an iceberg is near and should be constantly listened for. Visibility of icebergs. Despite their size, icebergs can be very difficult to see under certain circumstances, and the Mariner should invariably navigate with caution in waters in which they may be expected. In fog with sun shining an iceberg appears as a luminous white mass, but with no sun it appears close aboard as a dark mass, and the first signs may well be the wash of the sea breaking on its base. On a clear night with no moon icebergs may be sighted at a distance of 1 or 2 miles, appearing as black or white objects, but the ship may then be among the bergy bits and growlers often found in the vicinity of an iceberg. On a clear night, therefore, lookouts and radar operators should be particularly alert, and there should be no hesitation in reducing speed if an iceberg is sighted without warning. On moonlit nights icebergs are more easily seen provided the moon is behind the observer, particularly if it is high and full. At night with a cloudy sky and intermittent moonlight, icebergs are more difficult to see and to keep in sight. Cumulus or cumulo-nimbus clouds at night can produce a false impression of icebergs.
	Signs of drift ice
	There are two reliable signs of drift ice. Ice Blink whose characteristic light effects in the sky once seen, can never be mistaken, is one of these signs. On clear days, with the sky mostly blue, ice blink appears as a luminous yellow haze on the horizon in the direction of the ice. It is brighter below, and shades off upward, its height depending on the proximity of the ice field. On days with overcast sky, or low clouds, the yellow colour is almost absent, the ice blink appearing as a whitish glare on the clouds. Under certain conditions of sun and sky, both the yellowish and whitish glares may be seen simultaneously. It may sometimes be seen at night. Ice blink is observed some time before the ice itself appears over the horizon. It is rarely, if ever, produced by icebergs, but is always distinct over consolidated and extensive pack. Abrupt smoothing of the sea and the gradual lessening of the ordinary ocean swell is the other reliable sign, and a sure indication of drift ice to windward. Other likely signs include the presence of isolated fragments of ice which often point to the proximity of larger quantities. There is frequently a thick band of fog over the edge of drift ice. In fog, white patches indicate the presence of ice at a short distance. In the Arctic, if far from land, the appearance of walruses, seals and birds may indicate the proximity of ice. In the Antarctic, the Antarctic Petrel and Snow Petrel are said to indicate the proximity of ice--the former being found only within 400 miles of the ice edge, and the latter considerably closer to it. Sea surface temperatures give little or no indication of the near vicinity of ice. When, however, the surface temperature falls to +1° C, and the ship is not within one of the main cold currents, the ice edge should for safety be considered as not more than 150 miles distant, or 100 miles if there is a persistent wind blowing off the ice, since this will cause the ice temporarily to extend and become more open. A surface temperature of -0.5°C should generally be assumed to indicate that the nearest ice is not more than 50 miles away.
	Detection of ice by radar
	Though an invaluable aid, the limitations of radar in detecting ice must always be borne in mind. Absence of an indication of ice on the radar screen does not necessarily mean that there is no dangerous ice near the ship. The strength of the echo received from an iceberg depends as much on the inclination of its reflecting surfaces as on its size and range. When approaching the ice edge a continuous visual lookout is essential. Operators must be aware of the limitations given below and that less than full operating efficiency will greatly reduce the chance of detecting ice. The following conclusions have been reached from recent experience, but abnormal weather conditions may substantially reduce detection ranges. In a calm sea, ice formations of all sorts should be detected; from large icebergs at ranges of from 15 to 20 miles down to small growlers at a range of possibly 2 miles. However, bergy bits weighing several tons, and protruding up to 3 m out of the water, are unlikely to be detected at a range of more than 3 miles. As warning of ice may therefore be short, radar should be operated continuously in low visibility where ice is expected. In rough weather, it is unsafe to rely on radar when sea clutter extends beyond 1 mile, as insufficient warning will be given of the presence of growlers large enough to damage the ship, or of drift ice. Fields of concentrated hummocked ice should be detected in all sea conditions at a range of at least 3 miles. Ridges show clearly, but shadow areas behind ridges are liable to be mistaken for leads or the closed tracks of ships, and the large area of weak echoes given by a flat floe may be mistaken for a polynya. It is difficult to distinguish between 10/10 hummocked or rafted ice and 3/10 small floe and ice cakes. Large floes in the midst of brash ice will usually show on radar. A lead through static ice will not show on radar unless the lead is at least a mile wide and completely free from brash ice. Areas of open water and smooth floes appear very similar, but in an ice field the edge of a smooth floe is prominent, while the edge of open water is not. Snow, sleet and rain squalls can sometimes be detected. Lookouts can then be increased, or speed or course altered to avoid the squalls.
	Signs of open water
	Water sky, distinguished by dark streaks on the underside of low clouds, indicates the direction of leads or patches of open water. A dark band on the cloud at a high altitude indicates the existence along this line of small patches of open water which may connect with a larger distant area of open water. If low on the horizon, water sky may possibly indicate the presence of open water up to about 40 miles beyond the visible horizon. Dark spots in fog give a similar indication, but are only visible at shorter distances than reflections on clouds. The sound of a surge in the ice indicates the presence of large expanses of open water in the close vicinity.
	Effect of abnormal refraction
	Ice or open water in the distance may often be detected by super-refraction raising the horizon. The image of the ice or areas of open water, or a mixture of the two, may be seen as an erect or inverted image. Alternatively, both images may be seen at once, one above the other and usually in contact, in which case the erect image is the higher of the two. Allowance must be made for the fact that the refraction causing the mirage will increase the apparent dimensions of small ice, sometimes so greatly as to make small pieces appear like icebergs. The areas of open water are dark relative to the ice.
	THE MASTER'S DUTY REGARDING ICE
	Avoidance
	The International Convention for the Safety of Life at Sea (SOLAS), 1974, requires the Master of every ship, when ice is reported on or near his track, to proceed at a moderate speed at night or to alter course to pass well clear of the danger zone.
	Reports
	He is also required to make the following reports: On meeting dangerous ice: - Type of ice; - Position of the ice; - UTC (GMT) and date of observation. On encountering air temperatures below freezing associated with gale force winds causing severe ice accumulation on ships: - Air and sea temperatures; - Force and direction of the wind; - Position of the ship; -UTC (GMT) and date of observations.
	ICE REPORTS
	Extent
	Ice reports are available when ice is prevalent for the Arctic, Iceland, Baltic Sea, East coast of Canada, Gulf of Saint Lawrence, Gulf of Alaska, Bering Sea, Sea of Okhotsk, Sea of Japan and Antarctica. Some of these are Facsimile reports. Details of these reports and the radio stations transmitting them are given in Admiralty List of Radio Signals Vol 3. See ALRS Publications.
	International Ice Patrol
	The United States Coast Guard operates the International Ice Patrol, the cost being met by Signatory Nations to the 1974 SOLAS Convention. Its prime object is to warn ships of the extent and limits of icebergs and sea ice in the North Atlantic near the Grand Banks of Newfoundland. The service operates during the ice season from late February or early March to about the end of June. For details, see Admiralty List of Radio Signals Vol 5. See ALRS Publications.
	ICE ACCUMULATION ON SHIPS
	General information
	In certain conditions ice, formed of fresh water or sea water, accumulating on the hulls and superstructures of ships can be a serious danger. Ice accumulation may occur from three causes: - Fog, including fog formed by evaporation from a relatively warm sea surface, combined with freezing conditions; - Freezing drizzle, rain or wet snow. - Spray or sea water breaking over the ship when the air temperature is below the freezing point of sea water (about -2° C).
	Icing from fresh water
	From fog, drizzle, rain or snow, the weight of ice which can accumulate on the rigging may increase to such an extent that it is liable to fall and endanger those on deck. Radio and radar failures due to ice on aerials or insulators may be experienced soon after ice starts to accumulate. The amount of ice, however, is small compared with the amount which accumulates in rough weather with low temperatures, when heavy seas break over a vessel.
	Icing from sea water
	When the air temperature is below the freezing point of sea water and the ship is in heavy seas, considerable amounts of water will freeze on to the superstructure and those parts of the hull which are sufficiently above the waterline to escape being frequently washed by the sea. The amounts so frozen to surfaces exposed to the air will rapidly increase with falling air and sea temperatures, and have in extreme cases lead to the capsizing of vessels. The dangerous conditions are those in which strong winds are experienced in combination with air temperatures of about -2°C or below; freezing rain or snowfall increases the hazard. The rapidity with which ice accumulates increases progressively as the wind increases above force 6 and as the air temperature falls further below about -2°C. It also increases with decreasing sea temperatures. The rate of accumulation also depends on other factors, such as the ship's speed and course relative to the wind and waves, and the particular design of each vessel.
	Avoiding ice accumulation
	It will be appreciated that it is very difficult to forecast accurately the three variables involved. Furthermore, the region of icing often moves at such a rate that vessels cannot take evasion action unless warning of impending icing conditions is received. The Mariner is therefore advised to exercise all possible caution whenever gales are expected in combination with air temperatures of -2°C or below. These conditions are most likely to occur with winds from polar regions, but the direction may be any that will transport sufficient cold air. If these conditions are expected, the prudent course is to steer towards warmer conditions, or to seek shelter, as soon as possible. If unable to reach shelter or warmer conditions, it has been found best to reduce spray to a minimum by heading into the wind and sea at the slowest speed possible, or if weather conditions do not permit that, to run before the wind at the least speed that will maintain steerage way.
OPERATING IN ICE
General rule
Ice is an obstacle to any ship, even an ice breaker. The inexperienced ice navigator is advised to develop a healthy respect for the latent power and strength of ice in all its forms. However, well-found ships in capable hands can operate successfully in ice-covered waters. The first principle of successful passage through ice is to maintain freedom of manoeuvre. Once a ship becomes trapped, she goes wherever the ice goes. Operating in ice requires great patience and can be a tiring business with or without icebreaker escort. The long way round a difficult area whose limits are known, is often the quickest and safest way. In ice concentrations three basic ship handling rules apply: Keep moving, even if very slowly; Try to work with the ice movement and not against it; Excessive speed leads to ice damage.
Ice identification
Caution. Before attempting any passage through ice it is essential to determine its type, thickness, hardness, floe size and concentration. This can only be done visually. It is very easy and extremely dangerous to underestimate the hardness of ice. After a snow fall ice can be very difficult to identify. The utmost caution and experience is required then when making a passage through the ice. Ice is seldom uniform. There can be different types of ice in drift ice.
Changes in ice conditions
Ice moves continually under the influence of wind and current, floating ice is much influenced by the wind. With a change of wind, ice conditions can completely change, sometimes within hours. Ice fuses when the temperature fails below freezing. An area of separate ice floes and loose fragments can quickly turn into a solid mass of ice and pose serious problems, even for ice breakers. When practicable, a look-out from aloft will frequently detect distant leads and open water invisible from the bridge.
Considerations before entering ice
Ice should not be entered if an alternative, although longer, route is available. Before deciding to enter the ice the following factors need to be considered: - Type of ice; - Time of year, weather and temperature; - Area of operation; - Availability of icebreakers; - Vessel's ice class in relation to the type of ice expected; - State of hull, machinery and equipment, and quantity of bunkers and stores left; - Draught and depth of water over the propellor tips and the rudder; - Ice experience of the person in charge on the bridge. Thin new ice allows passage to be made through it by modern steel vessels on the original intended route. Thick first-year ice or old ice which cannot be negotiated considering the ice class of the vessel, requires the prudent Mariner to stop and wait until either conditions improve with a change of wind or tide, or an icebreaker is available.
Passage through ice Making an entry
The following principles govern entry into the ice: Where the existence of pressure is evident from hummocking and rafting, entry should never be attempted; The ice should be entered from leeward, if possible. The windward edge of an ice field is more compact than the leeward edge, and wave action is less on the leeward edge; The ice edge often has bights separated by projecting tongues. By entering at one of the bights, the surge will be found to be least. Ice should be entered at very low speed and at right angles to the ice edge to receive the initial impact, and once into the ice speed should be increased to maintain headway and control of the ship.
Drift ice
Ice masses of thick broken ice, especially those that bear signs of erosion by the sea on their upper surface, should be avoided. They have underwater spurs, extraordinarily strong and hardly affected by melting. If a large floe blocks the ship's intended course, no attempt should be made to break it unless it is very rotten. It is best to go round it, if possible, or to put the stem against it, to increase power until the floe is forced ahead and begins to swing to one side, when power should be reduced to allow it to pass clear. If collision with a floe cannot be avoided, it should be hit squarely with the stem. A glancing blow may damage the bow plating, and by throwing the ship off course cause another glancing blow from a nearby floe, or her stern to be swung into the ice damaging her rudder and propellors. If navigating in an extensive area of thin or light ice, the navigator, particularly in the Arctic, may suddenly come upon floes or fragments of hard ice that may be interspersed among the light ice. At night or in reduced visibility when passing through areas where ice is present, speed must be reduced or the ship stopped until the Mariner can see and identify the ice ahead of the ship. Navigation in ice after dark should not normally be attempted: if it is attempted, good searchlights are essential. Icebergs in an ice field. All forms of glacial ice and dirty ice broken away from coastal regions should be given a wide berth. Icebergs are usually current-driven while the ice field will have a wind drift component. With a strong current icebergs may travel upwind, when open water will be found to leeward, and piled up pressure to windward of the iceberg which may endanger a vessel unable to work clear. Similar conditions have been observed with a weak current and a strong wind, when the does overtaking an iceberg were heaped up to windward, while a lane of open water lay to leeward of the iceberg. In traversing drift ice, advantage may be taken of leads created by the movement of icebergs through a wind-compacted belt of ice.
Leads
Every opportunity should be taken to use leads through ice, but when not accompanied by an icebreaker, it is unwise to follow a shore lead with an onshore wind blowing. A ship stopped in ice close inshore should always be pointed to seaward unless it is intended to anchor.
Speed in ice
The force of the impact on striking ice, depends the vessel's tonnage and speed. It varies as the square of the speed. Speed in ice therefore requires careful consideration. If a vessel goes too slowly she risks being beset, if too fast she risks damage from collision with floes. Where concentrations of ice vary, and a ship passes from close ice, through a small patch of light ice or clear water, to more close ice, engine revolutions should be reduced on entering the more open patch. If revolutions are maintained, the ship will gather way as she passes through the clearer water, and be carrying too much way for re-entering the close ice.
Use of engines and rudder
Engines must be prepared to go full astern at any time. Propellors are the most vulnerable part of a ship. Ships should go astern in ice with extreme care, and always with the rudder amidships. If a ship is stopped by a heavy concentration of ice, the rudder should be put amidships and the engines kept turning slowly ahead. This will wash the ice astern clear, and enable the ship to come astern, after making certain that the propellors are clear of ice. If ice goes under a ship, speed should immediately be reduced to dead slow. Violent rudder movements should only be used in emergency. They may swing the stern into the ice, particularly in patches of clear water or leads during passage through the ice. Frequent use of the rudder, especially in the hard-over position, has the effect of slowing down the vessel's passage through ice. This can often be used to advantage to reduce speed without the loss of steerage way resulting from reducing the engine revolutions; Too much rudder, however, when pushing through ice or following an icebreaker, may bring the vessel to a complete stop.
Anchoring
In a heavy concentration of ice anchoring should be avoided. If ice is moving, its tremendous force may break the cable. When conditions permit anchoring, such as in light brash ice, rotten ice, or among widely scattered floes, the windlass and main engines should be kept at immediate notice, and the anchor weighed as soon as wind threatens to move ice on to the ship.
Ramming and backing
Forcing a passage through heavier ice to reach open water, or an area where ice is less heavy, may sometimes be justified. The method is to ram the ice to break it by sheer impact and weight, and then to back out of the ice into the water and broken ice astern. To avoid the risk of being embedded in the ice, the engines should be going astern before the vessel stops. By repeatedly carrying out this procedure, slow progress ahead can sometimes be made. It is not advisable, however, to continue forcing the passage unless the channel so made considerably exceeds the beam of the vessel to allow her to move freely out astern. Caution. The procedure is dangerous and should be used with the utmost discretion as heavy damage to a vessel can result. Only in extreme emergency should it be used by vessels with low or no ice class or those with a bulbous bow.
Beset
The most serious danger is from pressure of the ice which may crush the hull or nip off the ship's bottom. This risk is greater in ice concentrations of 7/10 or more. A ship beset in drift ice is at risk from drifting with the moving ice against icebergs, ice fronts, shoals and the shore: every precaution should be taken to avoid this situation. If the lee of an iceberg can be made whilst being swept along, it will provide safe shelter, but the possibility of the iceberg capsizing, or being held by a shoal, must be borne in mind. When a ship proceeding independently becomes beset it usually requires icebreaker assistance to free her. However, a ship can sometimes be freed by going full ahead and full astern alternately with full helm one way and then the other in order to swing her, this may loosen the ship sufficiently to enable her to move ahead through the ice. If the ship starts moving astern, the rudder must be dead amidships. Alternatively, ships in ballast can sometimes free themselves by pumping and transferring ballast from side to side, and it may need very little change in trim or list to release the ship. Other alternatives are: to take an anchor or warp to the ice astern, leading the cable through fairleads to the windlass, and to take the strain with the engines going full astern; or to lay out anchors on each beam and heave first on one and then on the other with the engines going full astern.
Dead reckoning
A careful reckoning should be kept of all alterations of course and speed together with the times at which they were made, so that a large scale plot of the ship's track can be maintained. The lack of information on tides and other factors usually prevents the most accurate dead reckoning from giving the exact position of the ship, but a carefully kept reckoning will considerably help to avoid errors. Icebergs, which can be regarded as stationary, can be of great value as temporary marks in maintaining the dead reckoning position. They may also mark shoals. In keeping the reckoning, the fundamental factors, speed and course, change continually and do not lend themselves to accurate calculation. Even if a gyro compass and automatic pilot are fitted, the speed relative to the ice is required, and this can rarely be measured continuously with accuracy. To check the resultant of the ship's course and speed through the ice, and the drift of the ice, every opportunity should be seized to obtain fixes or observed positions. The speed at any moment can be measured by timing the passage of an ice floe down a known length of the ship's side, like a Dutchman's log. The speed through the ice should be obtained as often as possible, or at least twice an hour.
Sights
Sights must be taken with great care, for in ice false horizons are frequently observed. It is normal in polar regions for the atmosphere to differ considerably from the standard, particularly near the sea surface. This affects both refraction and dip. Refraction variations of 2° or more are not uncommon and an extreme value of 5° has been reported. The sun has been known to rise as much as ten days before it was expected. A wise precaution is to apply corrections for air temperature and atmospheric pressure, particularly for altitudes of less than 5°. Because of the low temperature, the refraction correction for sextant altitudes may require to be taken from the appropriate table in the Nautical Almanac. If the horizon is covered with ice, it may still be used for celestial observations by subtracting the height of the ice on the horizon above the water from the height of eye of the observer: the maximum error this may cause is 4'. A bubble sextant; or a sextant used with an artificial horizon set up on the ice, will be found, however, to give better results. It should be remembered, however, that refraction elevates both the celestial body and the visible horizon, so that the error due to abnormal refraction is minimised if the visible horizon is used for observations.
Ends